Acylindrical hyperbolicity of automorphism groups and property $R_{\infty }$ for generalized Higman groups

Abstract

We consider the Higman groups $H_k$ ($k\ge 4$) and their generalizations studied by Martin and Horbez--Huang. We prove that the automorphism groups of the Horbez--Huang groups on at least five generators, as well as those of Martin's $4$-generated Higman-like groups, are acylindrically hyperbolic. It follows that the groups themselves are acylindrically hyperbolic, admit infinite-dimensional spaces of automorphism-invariant homogeneous quasimorphisms, have infinite $2$-generated simple characteristic quotients, and satisfy property $R_{\infty }$. We also prove, using Delzant's lemma, a general criterion ensuring that an individual automorphism has infinite Reidemeister number: if $G$ is a group, $\phi \in \mathrm{Aut}(G)$, the group ${\Gamma }_{\phi }=⟨\mathrm{Inn}(G),\phi ⟩$ acts by isometries on a Gromov-hyperbolic space, and the restricted action of $\mathrm{Inn}(G)$ has two independent loxodromic elements, then $R(\phi )=\infty$.

arXiv commentv2: Added results on the acylindrical hyperbolicity of automorphism groups and changed the title accordingly; expanded proofs and made minor corrections. 18 pages. To appear in the University of Tennessee Mathematical Journal